Though it is an unlikely event, it will happen given enough time. We might be unlucky and it may occur next year, or we may have to wait a 100,000 years for a major impact.

The number of Near Earth Objects (NEOs) is about half of that previously estimated. But the threat they pose is undiminished, according to a survey.

No known NEO has a Torino scale reading of more than one, which is a good thing as we have no coherent plan of action should a threat arise

Dr David Jewitt, University of Hawaii

The NEOs under scrutiny are between one kilometre and 10 km in size (0.6 to six miles). A 1 km object would be large enough to cause considerable local devastation but not global catastrophe.

A 10 km asteroid would devastate life on Earth.

It is thought that the asteroid that hastened the demise of the dinosaurs 65 million years ago was a 10 km object.

The 1908 impact at Tunguska in Siberia was caused by a comet 70m wide. It flattened trees over an uninhabited area of 1,000 square km.

700 asteroids

The Near Earth Asteroid Tracking (NEAT) project uses an automated telescope based on Hawaii.

Astronomers there surveyed 35,000 square degrees of the sky between March 1996 and August 1998 and detected 45 new NEOs.

From this, they deduce that the total number of potentially hazardous NEOs out there is about 700.

It is considerably less than previous estimates. Team leader David Rabinowitz, of Yale University, says in the journal Nature: "This decrease by a factor of two does not substantially affect the significance of the NEO hazard."

He adds that the new data means that we "can confidently predict the level of effort required to survey completely those NEOs capable of global devastation".

It is thought that at the current rate it will take about 15 years to track down 90% of them.

Slow progress

For some astronomers, this is fast enough, but others think it is far too slow and have been pressing the world's governments to take the NEO threat more seriously.

There are some signs that their arguments may be bearing fruit. Last week, the UK Government established a small taskforce to look into the threat.

About 100 NEOs are discovered each year and most pose no conceivable threat to the Earth at the moment.

But some researchers are worried that the media and the public will misinterpret their efforts and consider each new NEO a potential life-threatening object.

To counter this, last year astronomers devised a hazard-scale for NEOs called the Torino scale in which 10 is certain global devastation and zero no threat at all.

According to Dr David Jewitt of the University of Hawaii: "No known NEO has a Torino scale reading of more than one, which is a good thing as we have no coherent plan of action should a threat arise."

According to Dr Brian Marsden, of the Harvard Centre for Astrophysics, who has been called the custodian of asteroid orbits because of his co-ordinating role: "The NEO community as a whole still has a lot to learn about communicating information on possible NEO threats to the public."

Astronomers estimate that there is about a 1% chance that the Earth will be hit by a damaging NEO of 10 km size during the next million years. For smaller NEOs, say 300 metres in size, the probability of a strike in the next 100 years is also 1%.

Such a NEO would be the equivalent of a thousand megaton detonation and if it struck a city millions could die.